TOPIC 8 MORE ON WHILE LOOPS

Transcription

1 1 TOPIC 8 MORE ON WHILE LOOPS Notes adapted from Introduction to Computing and Programming with Java: A Multimedia Approach by M. Guzdial and B. Ericson, and instructor materials prepared by B. Ericson.

2 Outline 2 How to use logical operators How to use while loops in general How to do keyboard input

3 While Loops 3 Recall that the basic syntax for a while loop is: while (test) { body of loop } where test is a condition that is true or false body of the loop consists of the statements to be executed while the condition is true

4 While loops 4 We will now look at while loops that do not just involve counting Our tests will be more general than just checking the end value for a counter, as in a previous example: int total = 0; int counter = 1; while (counter <= 100) { total = total + counter; counter = counter + 1; }

5 Recall: Relational Operators 5 Relational operators > >= < <= ==!= Compare two operands of the same type Relational expressions Express a condition that evaluates to true or false Example: (counter <= 100) Sometimes we want to test for several conditions being true We can combine relational expressions using logical operators

6 Logical Operators 6 and operator && Used to check if several things are true Example: (x > 0) && (x <100) Expression evaluates to true if both (x >0) and (x < 100) Short circuiting : evaluation stops if the first condition turns out to be false Why does Java do this?

7 Logical Operators 7 or operator Used to check if at least one of several things is true Example: (x > 0) (x <100) Expression evaluates to true if either (x >0) or (x < 100) Java does short circuiting for also

8 Logical Operators 8 Exclusive or operator ^ Used to check if one and only one of the things is true Example: (x < 0) ^ (y <0) Expression evaluates to true if either (x < 0) or (y < 0) but not both Can this be short-circuited?

9 Logical Operators 9 not operator! Used to change the value of a condition to its opposite!true is false!false is true Example:!(x == y) Expression evaluates to true if (x == y) is false, evaluates to false if (x ==y) is true

10 Truth Tables for && and in Java 10 A B A&&B A B A B true true true true false false true any true false true true false any false false false false

11 Using && (and) and (or) 11 Check that a value is in a range Example: check for valid pixel color values Is some value between 0 and 255 inclusive? 0 <= x <= 255 is written in Java as 0 <= x && x <= 255 or x >= 0 && x <= 255 Check if at least one of several things is true Example: Is the color black or white?

12 Keyboard Input 12 The textbook provides a class called SimpleInput In the bookclasses folder It contains methods that make it easy for use to input data from the keyboard: getnumber for getting doubles getintnumber for getting integers getstring for getting strings They are class methods (not object methods)

13 Simple Input Methods 13 public static double getnumber(string message) The parameter message is the prompt message to display to the user The input window will keep appearing until a valid number type is input The number typed by the user is returned as a double

14 SimpleInput Methods 14 public static int getintnumber(string message) The number typed by the user is returned as an int public static String getstring(string message) The data typed by the user is returned as a String

15 While Loop Example 15 Suppose we want a user to enter a number between 0 and 255 inclusive. If the number entered is not in the correct range, the user should be asked again to enter the number: int number = -1; while ( number < 0 number > 255) { number = SimpleInput.getIntNumber("Enter a number between 0 and 255 inclusive: "); }

16 While Loop Example 16 What will happen if the user enters the number 300? the number -2? Why is the variable number initialized to -1 and not to 0?

17 Keyboard and while 17 Write a program that takes an image and changes the red value of all pixels to some specific value Exercise: write a method makered for the Picture class with the header public void makered(int redvalue) Sample main program: public class TestRed{ public static void main(string args[ ]) { int redvalue = 100; Picture p = new Picture(FileChooser.pickAFile()); p.explore(); p.makered(redvalue); p.explore(); } }

18 Continued 18 Now what if we wanted to see what the picture looks like with the red values set to 150? Or 200? We would have to change the statement int redvalue = 100; to int redvalue = 150; etc. We would have to recompile the program before we could run it again A more flexible way: ask the user to enter the new red value on the keyboard And also check that the input is valid!

19 Continued 19 public class TestRed { public static void main(string args[]) { int redvalue = -1; Picture p = new Picture(FileChooser.pickAFile()); p.explore(); while ( redvalue < 0 redvalue > 255) { redvalue = SimpleInput.getIntNumber("Enter a number between 0 and 255 inclusive: "); } p.makered(redvalue); p.explore(); } }

20 Summary 20 Relational Operators Logical Operators Simple Input From the Keyboard